This is a study of the ontogeny of cell-mediated immune mechanisms in the horse and is designed to elucidate the functional maturation of T lymphocyte subpopulations. Lymphoid tissues from various stages of gestation will be evaluated by culture with T and B lymphocyte mitogens, rosette-forming cell assays, and mixed lymphocyte cultures. The role of "blocking" maternal immunoglobulin in pregnancy-related anti-fetal immunity is to be investigated by mixed lymphocyte cultures of maternal and newborn lymphocytes taken before and after suckling, by cytotoxicity assays, and using maternal sera, colostrum, suckled foal sera, and placental extracts. Mechanisms of the immunobiology of spontaneous abortion thus clarified will apply to other species, including man.